Fiber modal noise mitigation by a rotating double scrambler

Fiber modal noise is a performance limiting factor in high-resolution spectroscopy, both with respect to achieving high signal-to-noise ratios or when targeting high-precision radial velocity measurements, with multi-mode fiber-fed high-resolution spectrographs. Traditionally, modal noise is reduced by agitating or "shaking" the fiber. This way, the light propagating in the fiber is redistributed over many different modes. However, in case of fibers with only a limited number of modes, e.g. at near-infrared wavelengths or in adaptive-optics assisted systems, this method becomes very inefficient. The strong agitation that would be needed stresses the fiber and could lead to focal ratio degradation, or worse, to damaging the fiber. As an alternative approach, we propose to make use of a classic optical double scrambler, a device that is already implemented in many high-precision radial-velocity spectrographs, to mitigate the effect of modal noise by rotating the scrambler's first fiber end during each exposure. Because of the rotating illumination pattern of the scrambler's second fiber, the modes that are excited vary continuously. This leads to very efficient averaging of the modal pattern at the fiber exit and to a strong reduction of modal noise. In this contribution, we present a prototype design and preliminary laboratory results of the rotating double scrambler.